In recent years, molecules of oligosaccharides have attracted attention as third chain life molecules following nucleic acids (DNA) and proteins. The human body is a huge cell society comprising about 60 trillion cells, and the surfaces of all the cells are covered with oligosaccharide molecules. For example, ABO blood groups are determined according to the difference of oligosaccharides over the surfaces of cells.
Oligosaccharides function in connection with the recognition of cells and interaction of cells and are key substances for the establishment of the cell society. Disturbances in the cell society lead, for example, to cancers, chronic diseases, infectious diseases and aging.
For example, it is known that when cells develop cancer, changes occur in the structure of oligosaccharides. It is also known that Vibrio cholerae, influenza virus, etc. ingress into cells and cause infection by recognizing and attaching to a specific oligosaccharide.
Clarification of oligosaccharide functions leads to development of pharmaceuticals and foods based on novel principles, contributing to the prevention and therapy of diseases, and a wide variety of applications are expected of oligosaccharides.
Oligosaccharides are much more complex than nucleic acids or proteins in structure because of the diversity of arrangements of simple sugars, modes or sites of linkages, lengths of chains, modes of branches and overall structures of higher order. Accordingly, biological information derived from the structures thereof is more diversified than is the case with nucleic acids and proteins. Although the importance of research on oligosaccharides has been recognized, the complexity and variety of structures thereof have delayed progress in the research on oligosaccharides unlike the studies on nucleic acids and proteins.
Many of proteins present on the surfaces of cell membranes or in serum have oligosaccharides attached thereto as described above. The molecules wherein oligosaccharides are combined covalently with proteins are termed glycoproteins, which can be divided into two groups according to the difference in the mode of linkage between the oligosaccharide and the protein. Oligosaccharides of one type are asparagine-linked oligosaccharides (N-glycoside linkage type) wherein an amino group of the side chain of asparagine (Asn) is linked with the oligosaccharide. Oligosaccharides of the other type are mucin-linked oligosaccharides (O-glycoside linkage type) wherein the oligosaccharide is linked with the alcohol of serine (Ser) or threonine (Thr). All the asparagine-linked oligosaccharides have a basic skeleton comprising five sugar residues, and are divided into subgroups of high-mannose type, composite type and mixture type, according to the kind of the nonreducing terminal sugar residue of the oligosaccharide linked. On the other hand, the mucin-liked oligosaccharides are divided into four groups according to the difference of the basic skeleton.
The process for preparing peptides which is presently in wide use is the solid-phase synthesis process developed by R. B. Merrifield in 1963. The solid-phase synthesis process is such that amino acids are linked to a solid phase called a resin to provide a lengthened peptide chain. When completely lengthened, the peptide chain is cut off from the solid phase to obtain the desired product. As an application of this process, a glycopeptide chain can be prepared by incorporating an amino acid having an oligosaccharide linked thereto into the peptide chain to be lengthened.
Accordingly, glycopeptide chains are widely prepared by using amino acid-linked oligosaccharides wherein an oligosaccharide is linked with Asn or Ser(Thr) for the preparation of peptides. However, there are only a few examples of chemically preparing peptide chains having a great sugar chain despite of technical progress in chemical synthesis.
One of the problems to be encountered is insufficient absolute amounts of oligosaccharides to be linked with the asparagine residue. Methods of obtaining oligosaccharides include isolation of oligosaccharides only from glycoproteins which are present in the living body. However, hydrazine for use in cutting off oligosaccharides from glycoproteins is hazardous, presenting difficulty in preparing large quantities of oligosaccharides. Further there are in the living body many oligosaccharides which closely resemble in structure, and it is difficult to obtain a single oligosaccharide only. Further since decomposition of hydrazine releases the oligosaccharide from the asparagine residue, there arises a need to link the released oligosaccharide with the asparagine residue again, hence an increased number of steps needed.
In chemically synthesizing oligosaccharides, there are examples of preparing oligosaccharides wherein about 10 sugar residues are linked, whereas many of these cases are such that the desired oligosaccharide can be prepared in an amount of only several milligrams during one year. For this reason, difficulties are encountered in chemically preparing oligosaccharides.
The second of the problems is involved in the treatment conducted with use of TFA (trifluoroacetic acid) for cutting off the peptide chain from the solid phase. For example, sialic acid present at the nonreducing terminals of oligosaccharides is readily hydrolyzed under an acid condition, so that there is the possibility that the TFA treatment will cut off sialic acid from the glycopeptide prepared. Accordingly, there is almost no case wherein oligosaccharides having sialic acid are used for solid-phase synthesis. To solve this problem, a process has been reported wherein sialic acid is transferred to an oligosaccharide with sialic acid transferase after peptide synthesis. Although useful for introducing sialic acid, this process still has the problem that difficulty is encountered in preparing glycopeptides in large quantities because the transferase is expensive.
As will be described below, however, the present invention has made it possible to artificially prepare glycopeptides in large amounts. Accordingly, it becomes possible to industrially introduce sialic acid or derivatives thereof into oligosaccharides using the sialic acid transferase.
Although there are naturally occurring oligosaccharides which have sialic acid linked thereto, oligosaccharides having sialic acid derivatives linked thereto are naturally unavailable. Thus, it is through the use of the sialic acid transferase that sialic acid derivatives can be introduced into oligosaccharides in any way.
An object of the present invention is to provide a process capable of artificially and easily preparing a large amount of a glycopeptide having at least one asparagine-linked oligosaccharide or mucin-linked oligosaccharide at a desired position of the peptide chain thereof.
Another object of the present invention is to provide a process for easily preparing a sialylglycopeptide which comprises an asparagine-linked oligosaccharide having sialic acid and wherein the sialic acid is not cut off from the glycopeptide by an acid treatment.
Another object of the present invention is to provide a process for artificially and easily preparing a large quantity of a glycopeptide having at least one of various novel asparagine-linked oligosaccharides at a desired position of the peptide chain thereof, with sugar residues removed therefrom as desired.
Another object of the present invention is to provide a process for preparing a glycopeptide having sialic acid or a derivative thereof introduced into the peptide with use of a sialic acid transferase.
Still another object of the invention is to provide glycopeptides which is obtainable by the above processes for preparing glycopeptides.